Mixer and differential amplifier having bandpass frequency selectivity

ABSTRACT

A mixer and a differential amplifier are formed using simple circuit configurations such that the cutoff frequencies thereof can be easily changed. Each of the mixer and the differential amplifier includes an NMOS transistor to which an RF signal is input, NMOS transistors to which an LO− signal and an LO+ signal are respectively input from a local oscillator, and two parallel resonant circuits each serving as an output load and including an active inductor, a capacitor, and a resistor.

BACKGROUND OF THE INVENTION

[0001] 1. Field of Invention

[0002] The present invention relates to a mixer for mixing an AC signalwith a reference signal having a particular frequency, and also to adifferential amplifier for amplifying the difference between two signalsand outputting a resultant amplified differential signal.

[0003] 2. Description of Related Art

[0004] In an RF (Radio Frequency) receiving circuit, a received RFsignal is mixed by a mixer with an LO (Local Oscillator) signal and theRF signal is down-converted into an IF (Intermediate Frequency) signal.

[0005]FIG. 7 illustrates a mixer in an RF receiving circuit.

[0006]FIG. 8 illustrates an exemplary process in which the mixer shownin FIG. 7 down-converts an RF signal into an IF signal.

[0007]FIG. 7 illustrates an RF signal serving as a carrier signal and anLO signal supplied from a local oscillator (not shown) applied to themixer 101. The mixer 101 mixes the RF signal and the LO signal andoutputs an IF signal as shown in FIG. 8. Thus, the RF signal isdown-converted into the IF signal.

[0008] When it is required to remove undesirable signal components infrequency bands other than the IF frequency band from the IF signalobtained via the down conversion, a bandpass filter is generallypositioned at a stage following the mixer. FIG. 9 illustrates a mixerand a bandpass filter. FIG. 10 illustrates an exemplary process in whichundesirable signal components are removed from an IF signal by thebandpass filter.

[0009] As shown in FIG. 10, when there are signals A1 and B1 at bothsides of an RF signal, the RF signal including the signals A1 and B1 andthe LO signal are applied to the mixer 101 shown in FIG. 9. As a result,in addition to the IF signal, signals A2 and B2 are output from themixer 101. If signals A2 and B2 are passed through the bandpass filter,the signals A2 and B2 are attenuated into signals A3 and B3. Thus, theirinfluence on the IF signal is reduced.

[0010]FIG. 11 illustrates a circuit configuration of the bandpass filtershown in FIG. 9. FIG. 12 illustrates the frequency characteristic of thebandpass filter shown in FIG. 11.

[0011] As shown in FIG. 11, the bandpass filter 102 is formed of passiveelements including capacitors 102_1 and 102_4 and resistors 102_2 and102_3. As shown in FIG. 12, the bandpass filter 102 has cutofffrequencies f1 and f2 determined by the values of the passive elements.When the capacitors 102_1 and 102_4 have capacitance C1 and C2, and theresistors 102_2 and 102_3 have resistance R1 and R2, the cutofffrequency fl is given by the equation: $\begin{matrix}{{f1} = \frac{1}{2\quad \pi \sqrt{{C1} \cdot {R1}}}} & (1)\end{matrix}$

[0012] and the cutoff frequency f2 is given by the equation:$\begin{matrix}{{f2} = \frac{1}{2\quad \pi \sqrt{{C2} \cdot {R2}}}} & (2)\end{matrix}$

[0013] The bandpass filter 102 passes frequency components within aparticular band determined by the cutoff frequencies f1 and f2.

[0014]FIG. 13 illustrates a circuit configuration of a bandpass filter,configured differently from the bandpass filter shown in FIG. 11. FIG.14 illustrates the frequency characteristic of the bandpass filter shownin FIG. 13.

[0015] The bandpass filter 103 shown in FIG. 13 is an active bandpassfilter including capacitors 103_1 and 103_4, resistors 103_2 and 103_3,and an operational amplifier 103_5. As with the bandpass filter 102shown in FIG. 11, the bandpass filter 103 also has cutoff frequencies f3and f4 determined by values of the passive elements, and the bandpassfilter 103 passes frequency components within a particular banddetermined by the cutoff frequencies f3 and f4.

[0016]FIG. 15 illustrates a biquad bandpass filter. FIG. 16 illustratesa circuit configuration of a transconductor amplifier used in the biquadbandpass filter.

[0017] The biquad bandpass filter 104 shown in FIG. 15 is a bandpassfilter using the Gm-C technology comprising transconductor amplifiers(OTAs: Operational Transconductance Amplifiers) 104_1, 104_2, and 104_3,capacitors 104_4, 104_5, 104_6, and 104_7, and a resistor 104_8. Thecapacitors 104_4, 104_5, 104_6, and 104_7 all have equal capacitance C,and the resistor 104_8 has resistance R.

[0018] The transconductor amplifier 104_1 includes, as shown in FIG. 16,NMOS transistors 104_11, 104_12, 104_13, 104_14, 104_15, 104_16, 104_17,104_18, and 104_19, constant current sources 104_20, 104_21, 104_22,104_23, and resistors 104_24 and 104_25. Signals IN+ and IN−, which aredifferent in phase by 180° from each other, are applied to the NMOStransistors 104_11 and 104_12, respectively. An external voltage signalVf is applied to the NMOS transistor 14_19. The transconductance gm ofthe transconductor amplifier 104_1 varies depending on the value of theexternal voltage signal Vf applied to the NMOS transistors 104_19. Thetransconductance gm is given by the equation:

gm=β(Vf−Vs−Vt)

[0019] wherein β is the feedback factor of the NMOS transistor 104_19,Vs is equal to Vs2 (when Vs1>Vs2) or Vs1 (when Vs1<Vs2) (Vs1 and Vs2 aresource and drain voltages, respectively, of the NMOS transistor 104_19),and Vt is the threshold voltage of the NMOS transistor 104_19.

[0020] Although the circuit configuration has been described above onlyfor the transconductor amplifier 104_1, the transconductor amplifiers104_2 and 104_3 also have a similar circuit configuration.

[0021]FIG. 17 illustrates the frequency characteristic of the biquadbandpass filter shown in FIG. 15.

[0022] The frequency characteristic of this biquad bandpass filter 15shown in FIG. 17 is variable. More specifically, the cutoff frequenciesf01 and f02 can be varied by varying the external voltage signal Vfthereby varying the transconductance gm of the transconductor amplifiers104_1, 104_2, and 104_3. For example, when the external voltage signalVf applied to the transconductor amplifier 104_2 is varied, the centerfrequency f0 shown in FIG. 17 is given by the equation:

f 0=gm2/2πC

[0023] where gm2 is the transconductance of the transconductor amplifier104_2.

[0024] On the other hand, the difference between the cutoff frequencyf01 and the cutoff frequency f02 is given by the equation: Δf=gm2×R.

[0025] In the bandpass filters 102 and 103 shown in FIGS. 11 and 13,respectively, their cutoff frequencies are determined by the values ofpassive elements. This means that, to change the cutoff frequencies, thepassive elements themselves must be changed. To change the values ofpassive elements formed on a semiconductor chip using CMOS technology orthe like, it is required to change the layout of the passive elements ofthe semiconductor chip. The change in the layout needs a long time andhigh cost and thus the change results in great disadvantages inproduction or development. Another problem is that passive elementsoccupy large areas on the semiconductor chip.

[0026] Although the biquad bandpass filter 104 shown in FIG. 15 has theadvantage that the cutoff frequencies can be controlled by the externalvoltage signal, the biquad bandpass filter 104 has the disadvantage thatthe circuit configuration of the transconductor amplifiers 104_1, 104_2,and 104_3 is complicated, needs a large number of transistors, and thenneeds a large-scale circuit.

SUMMARY OF THE INVENTION

[0027] In view of the above, it is an object of the present invention toprovide a mixer and a differential amplifier which have simple circuitconfigurations and which allow the cutoff frequency to be easilychanged.

[0028] According to an aspect of the present invention, a mixer isprovided for mixing an AC signal with a reference signal having aparticular frequency, wherein the mixer includes a parallel resonantcircuit including an active inductor and serving as an output load.

[0029] Preferably, the AC signal is an RF signal and the referencesignal is an output signal of a local oscillator, the frequency of theoutput signal being different by a particular value from the frequencyof the RF signal.

[0030] Preferably, the active inductor includes two transconductancecircuits and a capacitor such that the inductance of the active inductoris set by the transconductance of the two transconductance circuits andthe capacitance of the capacitor.

[0031] Preferably, the inductance of the active inductor can bearbitrarily varied by controlling the transconductance of the twotransconductance circuits in response to an external signal.

[0032] Preferably, the parallel resonant circuit comprises an activeinductor, a capacitor, and a resistor that are connected in parallel.

[0033] Preferably, the parallel resonant circuit has bandpass frequencyselectivity given by the expression:${\frac{1}{2\quad \pi \sqrt{LC}} + {\frac{R}{2} \cdot \sqrt{\frac{C}{L}}}} \geq f \geq {\frac{1}{2\quad \pi \sqrt{LC}} - {\frac{R}{2} \cdot \sqrt{\frac{C}{L}}}}$

[0034] where L is the inductance of the active inductor, C is thecapacitance of the capacitor, R is the resistance of the resistor, and fis the resonant frequency of the parallel resonant circuit.

[0035] According to another aspect of the present invention, there isprovided a differential amplifier for amplifying the difference betweentwo input signals and outputting a resultant amplified differentialsignal, wherein the differential amplifier includes a parallel resonantcircuit including an active inductor and serving as an output load.

[0036] Preferably, the active inductor includes two transconductancecircuits and a capacitor such that the inductance of the active inductoris set by the transconductance of the two transconductance circuits andthe capacitance of the capacitor.

[0037] Preferably, the inductance of the active inductor can bearbitrarily varied by controlling the transconductance of the twotransconductance circuits in response to an external signal.

[0038] Preferably, the parallel resonant circuit includes an activeinductor, a capacitor, and a resistor that are connected in parallel.

[0039] Preferably, the parallel resonant circuit has bandpass frequencyselectivity given by the expression:${\frac{1}{2\quad \pi \sqrt{LC}} + {\frac{R}{2} \cdot \sqrt{\frac{C}{L}}}} \geq f \geq {\frac{1}{2\quad \pi \sqrt{LC}} - {\frac{R}{2} \cdot \sqrt{\frac{C}{L}}}}$

[0040] where L is the inductance of the active inductor, C is thecapacitance of the capacitor, R is the resistance of the resistor, and fis the resonant frequency of the parallel resonant circuit.

[0041] As described above, each of the mixer and the differentialamplifier according to the present invention has a resonant circuitincluding an active inductor and serving as an output load. The activeinductor includes a transconductance circuit, which is constructed in asimple form as will be described later with reference to specificembodiments. The inductance L of the active inductor can be arbitrarilyvaried by controlling the transconductance of the transconductancecircuit in response to an external signal, thereby easily varying thecutoff frequencies of the bandpass frequency selectivity. Each of themixer and the differential amplifier can be formed on a semiconductorchip with a smaller size than is needed for a conventional bandpassfilter using passive elements.

BRIEF DESCRIPTION OF THE DRAWINGS

[0042]FIG. 1 is a circuit diagram of a mixer according to an embodimentof the present invention;

[0043]FIG. 2 is a diagram showing an equivalent circuit of an activeinductor used in the mixer shown in FIG. 1;

[0044]FIG. 3 is a graph showing the impedance-frequency characteristicof a parallel resonant circuit;

[0045]FIG. 4 is a graph showing the characteristic of a bandpass filterof the mixer shown in FIG. 1;

[0046]FIG. 5 is a circuit diagram showing a circuit configuration of theactive inductor used in the parallel resonant circuit of the mixer shownin FIG. 1;

[0047]FIG. 6 is a circuit diagram of a differential amplifier accordingto an embodiment of the present invention;

[0048]FIG. 7 is a diagram showing a mixer used in an RF receivingcircuit;

[0049]FIG. 8 is a diagram showing a manner in which an RF signal appliedto the mixer shown in FIG. 7 is down-converted into an IF signal;

[0050]FIG. 9 is a diagram showing a mixer and a bandpass filter;

[0051]FIG. 10 is a diagram showing a manner in which undesirable signalsother than an IF signal are removed by a bandpass filter;

[0052]FIG. 11 is a diagram showing the circuit configuration of thebandpass filter shown in FIG. 9;

[0053]FIG. 12 is a graph showing the frequency characteristic of thebandpass filter shown in FIG. 11;

[0054]FIG. 13 is a circuit diagram of another bandpass filter having aconfiguration different from that of the bandpass filter shown in FIG.11;

[0055]FIG. 14 is a graph showing the frequency characteristic of thebandpass filter shown in FIG. 13;

[0056]FIG. 15 is a circuit diagram of a biquad bandpass filter;

[0057]FIG. 16 is a circuit diagram of a transconductor amplifier used inthe biquad bandpass filter; and

[0058]FIG. 17 is a graph showing the frequency characteristic of thebiquad bandpass filter shown in FIG. 15.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0059] The present invention is described in further detail below withreference to preferred embodiments.

[0060]FIG. 1 is a circuit diagram of a mixer according to an embodimentof the present invention.

[0061] The mixer 10 shown in FIG. 1 is a single-balanced mixer formed ona semiconductor chip using CMOS technology or the like. The mixer 10includes an NMOS transistor 11 to which an RF signal is input, an NMOStransistor 12 to which an LO− signal is input from a local oscillator,and an NMOS transistor 13 to which an LO+ signal different in phase by180° from the LO− signal is input.

[0062] In the mixer 10, a parallel resonant circuit 14 serving as anoutput load of the mixer 10 is disposed between a power supply V_(DD)and the NMOS transistor 12, and a parallel resonant circuit 15 servingas an output load is disposed between the power supply V_(DD) and theNMOS transistor 13.

[0063] The parallel resonant circuit 14 includes an active inductor14_1, a capacitor 14_2, and a resistor 14_3. The parallel resonantcircuit 15 is configured in a similar manner as the parallel resonantcircuit 14 and includes an active inductor 15_1, a capacitor 15_2, and aresistor 15_3.

[0064]FIG. 2 is a diagram showing an equivalent circuit of the activeinductor 14-1 used in the mixer 10 shown in FIG. 1.

[0065] Although FIG. 2 shows the circuit configuration and theequivalent circuit only for the active inductor 14_1, the activeinductor 15_1 also has a similar circuit configuration and equivalentcircuit.

[0066] The active inductor (also called a gyrator) 14_1 shown in FIG. 2includes two transconductance (gm) circuits 14_11 and 14_12 and acapacitor 14_13. This active inductor 14_1 is equivalent to an inductorhaving inductance L given by the equation: $\begin{matrix}{L = \frac{C_{L}}{{gm1} \cdot {gm2}}} & (3)\end{matrix}$

[0067] where gm1 and −gm2 are transconductance of the transconductancecircuits 14_11 and 14_12, respectively, and C_(L) is the capacitance ofthe capacitor 14_13.

[0068] As will be described later, the inductance L of the activeinductor 14_1 can be set to an arbitrary value by adjusting thetransconductance gm1 and gm2 of the transconductance circuits 14_11 and14_12, by controlling the external signal, or by adjusting thecapacitance C_(L) of the capacitor 14_13.

[0069] The impedance Za of the parallel resonant circuit 14 (and alsothe parallel resonant circuit 15) used as the load of the mixer 10 isgiven by the equation: $\begin{matrix}{{Za} = \frac{1}{\frac{1}{R} + \frac{1}{j\quad 2{fL}} + {j\quad 2\quad \pi \quad c}}} & (4)\end{matrix}$

[0070]FIG. 3 shows the impedance-frequency characteristic of theparallel resonant circuit 14.

[0071] In FIG. 3, the horizontal axis indicates the frequency and thevertical axis indicates the impedance. In this parallel resonant circuit14, the impedance has a maximum value Zmax at a frequency of ½π{squareroot}{square root over (LC)} and has a value of Zmax/{squareroot}{square root over (2)} at a frequency lower by R/2{squareroot}{square root over (C/L)} and also at a frequency higher byR/2{square root}{square root over (C/L)} than the frequency at which theimpedance has the maximum value Zmax. Such frequency selectivity of theoutput load of the mixer 10 causes the output signal of the mixer 10 tohave frequency selectivity, and thus the mixer 10 has bandpass frequencyselectivity.

[0072]FIG. 4 shows the bandpass frequency characteristic of the mixershown in FIG. 1.

[0073] In FIG. 4, the horizontal axis indicates the frequency and thevertical axis indicates the gain. Herein, the cutoff frequencies f1 andf2 are given by the equations: $\begin{matrix}{{f1} = {\frac{1}{2\quad \pi \sqrt{LC}} - {\frac{R}{2} \cdot \sqrt{\frac{C}{L}}}}} & (5) \\{{f2} = {\frac{1}{2\quad \pi \sqrt{LC}} + {\frac{R}{2} \cdot \sqrt{\frac{C}{L}}}}} & (6)\end{matrix}$

[0074] In the mixer 10 according to the present embodiment, as will bedescribed later, the transconductance gm of the transconductance circuitof each of the active inductors 14_1 and 15_1 can be arbitrarilyadjusted by the external signal thereby arbitrarily varying theinductance L. This makes it possible to vary the cutoff frequencies ofthe bandpass filter within the ranges given by equations (5) and (6),respectively.

[0075]FIG. 5 shows a circuit configuration of the active inductor usedin the parallel resonant circuit of the mixer shown in FIG. 1.

[0076] The active inductor 14_1 shown in FIG. 5 includes, as describedearlier with reference to FIG. 2, the transconductance circuits 14_11and 14_12 and the capacitor 14_13. The transconductance circuit 14_11includes a constant current source 14_11 a, PMOS transistors 14_11 b and14_1 c, and NMOS transistors 14_11 d and 14_11 e. Abias voltage Vbias isapplied as the external signal from the outside of the chip to the PMOStransistor 14_11 c. The transconductance circuit 14_12 includes aconstant current source 14_12 a, and an NMOS transistor 14_12 b. Thetransconductance gm of the transconductance circuit 14_11 and that ofthe transconductance circuit 14_12, forming the active inductor 14_1,are equal to the transconductance gm of the PMOS transistor 14_11 c andthat of the NMOS transistor 14_12 b, respectively. Therefore, bycontrolling the bias current flowing through the respectivetransconductance circuits 14_11 and 14_12 according to the bias voltageVbias, the transconductance gm of the PMOS transistor 14_1 c and that ofthe NMOS transistor 14_12 b can be controlled arbitrarily. The activeinductor 15_1 has a circuit configuration similar to that of the activeinductor 14_1. Although in the present embodiment, the bias currentspassed through the transconductance circuits 14_11 and 14_12 arecontrolled by the bias voltage Vbias, the bias currents passed throughthe transconductance circuits 14_11 and 14_12 may be directly controlledfrom the outside of the chip.

[0077] Use of the active inductors 14_1 and 15_1 allows achievement ofhigh inductance using a small number of transistors, and thus it becomespossible to reduce the chip size compared with the chip sizes requiredfor the bandpass filters shown in FIGS. 11, 13 and 15.

[0078] Although in the present embodiment, the invention is applied to asingle balanced mixer, the invention may also be applied to other typesof mixers, as long as the mixers include a parallel resonant circuitwhich includes an active inductor and which serves as an output load.

[0079]FIG. 6 is a circuit diagram of a differential amplifier accordingto an embodiment of the present invention.

[0080] The differential amplifier 20 shown in FIG. 6 includes a constantcurrent source 21, NMOS transistors 22 and 23 to which signals IN− andIN+, separated in phase by 180° from each other, are applied, a parallelresonant circuit 24 serving as an output load disposed between a powersupply V_(DD) and the NMOS transistor 22, and a parallel resonantcircuit 25 serving as an output load disposed between the power supplyV_(DD) and the NMOS transistor 23.

[0081] The parallel resonant circuit 24 includes an active inductor24_1, a capacitor 24_2, and a resistor 24_3. Similarly, the parallelresonant circuit 25 includes an active inductor 25_1, a capacitor 25_2,and a resistor 25_3. The operations and functions of those parallelresonant circuits 24 and 25 are similar to those of the parallelresonant circuits 14 and 15 described above, and thus a furtherdescription thereof is not given herein. Use of the parallel resonantcircuits 24 and 25 as the output loads of the differential amplifier 20causes the differential amplifier 20 to have bandpass frequencyselectivity similar to that of the mixer 10 described above.

[0082] Furthermore, use of parallel resonant circuits as output loads inthe mixer 10 or the differential amplifier 20 according to the presentinvention makes it possible to control the cutoff frequency of thebandpass frequency selectivity by the external signal. This allowsreductions in production costs and the development period. Furthermore,compared with conventional bandpass filters using passive elements, asmaller size of the semiconductor chip can be realized. This allows areduction in cost for the semiconductor chip.

[0083] As described above, the mixer and the differential amplifieraccording to the present invention has the advantage that the cutofffrequencies can be easily changed and they can be formed to be simple incircuit configuration.

[0084] While particular embodiments have been described, alternatives,modifications, variations, improvements and substantial equivalents thatare or may be presently unforeseen may arise to Applicant or othersskilled in the art. Accordingly, the amended claims as filed and as theymay be amended are intended to embrace all such alternatives,modifications, variations, improvements and substantial equivalents.

What is claimed is:
 1. A mixer for mixing an AC signal with a referencesignal having a particular frequency, comprising a parallel resonantcircuit including an active inductor and serving as an output load.
 2. Amixer according to claim 1, wherein the AC signal is an RF signal andthe reference signal is an output signal of a local oscillator, thefrequency of the output signal being separated by a particular valuefrom the frequency of the RF signal.
 3. A mixer according to claim 1,wherein the active inductor comprises two transconductance circuits anda capacitor, and the inductance of the active inductor is controlled bythe transconductance of the two transconductance circuits and thecapacitance of the capacitor.
 4. A mixer according to claim 3, whereinthe inductance of the active inductor can be arbitrarily varied bycontrolling the transconductance of the two transconductance circuits inresponse to an external signal.
 5. A mixer according to claim 1, whereinthe parallel resonant circuit comprises an active inductor, a capacitor,and a resistor that are connected in parallel.
 6. A mixer according toclaim 5, wherein the parallel resonant circuit has bandpass frequencyselectivity given by the expression:${\frac{1}{2\quad \pi \sqrt{LC}} + {\frac{R}{2} \cdot \sqrt{\frac{C}{L}}}} \geq f \geq {\frac{1}{2\quad \pi \sqrt{LC}} - {\frac{R}{2} \cdot \sqrt{\frac{C}{L}}}}$

where L is the inductance of the active inductor, C is the capacitanceof the capacitor, R is the resistance of the resistor, and f is theresonant frequency of the parallel resonant circuit.
 7. A differentialamplifier for amplifying the difference between two input signals andoutputting a resultant amplified differential signal, comprising aparallel resonant circuit including an active inductor and serving as anoutput load.
 8. A differential amplifier according to claim 7, whereinthe active inductor includes two transconductance circuits and acapacitor, and the inductance of the active inductor is controlled bythe transconductance of the two transconductance circuits and thecapacitance of the capacitor.
 9. A differential amplifier according toclaim 8, wherein the inductance of the active inductor can bearbitrarily varied by controlling the transconductance of the twotransconductance circuits in response to an external signal.
 10. Adifferential amplifier according to claim 7, wherein the parallelresonant circuit comprises an active inductor, a capacitor, and aresistor that are connected in parallel.
 11. A differential amplifieraccording to claim 7, wherein the parallel resonant circuit has bandpassfrequency selectivity given by the expression:${\frac{1}{2\quad \pi \sqrt{LC}} + {\frac{R}{2} \cdot \sqrt{\frac{C}{L}}}} \geq f \geq {\frac{1}{2\quad \pi \sqrt{LC}} - {\frac{R}{2} \cdot \sqrt{\frac{C}{L}}}}$

where L is the inductance of the active inductor, C is the capacitanceof the capacitor, R is the resistance of the resistor, and f is theresonant frequency of the parallel resonant circuit.